The invention relates generally to power conversion systems and more specifically to a method and system for providing high quality output power.
Many devices, such as power supply systems, include power conversion systems. A power conversion system usually comprises converter systems used to convert an input voltage which may be fixed frequency, variable frequency, or dc to a desired converter output voltage. The output voltage and frequency may be fixed or variable. A converter system usually includes several switches such as insulated gate bipolar transistors (IGBTs) that are switched at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads via transformers. Loads as used herein is intended to broadly include motors, power grids, and resistive loads, for example.
It is often desired that the converter output voltage be of high quality. More specifically, it is desired to maintain harmonic components in the converter output voltage at a minimum level. Such a state is usually attained by switching the converter systems at high frequencies.
In several applications, such as compressor drives for gas turbine driven power generators, high speed electrical machines are directly integrated to the compressor or turbine shaft without a gearbox. However, the power converter that interfaces with the machine is usually not designed to handle high power at high fundamental frequencies due to limited switching frequency of the power converter.
Typically, in power conversion systems at the megawatt level, the switching frequency of the converter is limited to few hundred cycles per second. The reduced switching frequency results in higher harmonic components in the output voltage.
The limitation in switching frequency also limits the fundamental frequency that can be achieved for high power. One way to address the limitation is to use multilevel converter topologies such as the neutral point clamped (NPC) topology, flying capacitor (FC) topology, or the H-bridge series cell (HSC) topology. The HSC topology has a higher degree of modularity allowing for increased levels in the output voltage thus allowing the flexibility of increasing the fundamental frequency. However, the HSC topology results in single phase power being reflected as double frequency ripple on the dc link. This effect results in a large value of dc link capacitance and also affects the control of the front-end/grid-side converter.
One method for reducing harmonic components in the output voltage is to increase the switching frequency of the converter system. When switching frequency is increased, there is a proportionate increase in size of heat-removal components such as heat-sinks.
Therefore, what is desired is a power conversion system that generates a high quality output voltage with minimum harmonic components while operating at a high efficiency.